Published July 14, 2014

buszaki-profileDrs Antal Berényi and György Buzsáki recently published a paper titled "Large-scale, high-density (up to 512 channels) recording of local circuits in behaving animals."

This paper describes a system that allows high-channel-count recordings from a small volume of neuronal tissue using a lightweight signal multiplexing headstage that permits free behavior of small rodents. The system integrates multishank, high-density recording silicon probes, ultraflexible interconnects, and a miniaturized microdrive. These improvements allowed for simultaneous recordings of local field potentials and unit activity from hundreds of sites without confining free movements of the animal.

NeuroNexus caught up Dr. Buzsáki to discuss recording using probes with very high densities of recording sites, such as the NeuroNexus Buzsaki256 probe.

Q: In the introduction to the paper it is stated that “monitoring a statistically representative fraction of neurons of the investigated circuits in behaving animals is a prerequisite for understanding neuronal computation.” In your opinion, what is the practical lower limit for the number of channels required to reach that “statistically representative fraction”? In a perfect world, is there an upper limit on how many sites you would like to record from?

This is of course a super important question and there is not a good answer to it. If one records from a homogenous population (eg., locus coeruleus), the fraction may be small and perhaps 1-5% is sufficient. On the other hand, in the neocortex, there is an enormous diversity of cell classes, and some types of interneurons are very rare. Thus, if one wants to have a ‘representative’ snapshot of how neurons interact recordings from even the rarest ones are needed. An additional complication for sampling eg., pyramidal neurons of the cortex is that their firing rates show a very strongly skewed (typically log-normal) distribution. While only 10% of the neurons do half of the job, understanding the remaining half requires sampling from very large numbers of neurons. The more the better.

Q: The Buzsaki256 probe has 256 sites within the active recording area that spans 1.6 mm (along the shanks) by 2.1 mm. In the discussion of the paper it is stated that “Denser recording sites without increasing the shank volume is desirable,” and you look on the horizon to probes with “>1000 site counts and 20 um site spacing.” What site density would you consider sufficient for unit recordings and cell clustering? What is the optimal minimum site spacing, beyond which you see no further benefit?

Designing an optimum probe depends on the scientific question. For reliably separating cortical pyramidal neurons our estimate is that 15-20 µm spacing is a good compromise between separability of neurons and probe size. One would like to have more but without increasing probe width and depth. The size of the probe is very critical. Most current NeuroNexus probes are too wide and far from the ideal. For the new 1000 site probes, I specified the 20 µm spacing out of compromise. My favorite probe for exploring cortical circuits would have <60 µm shanks, 20 µm spacing, staggered on the sides, and contain 250-300 sites to cover the entire depth of the neocortex. Multiple shanks with 150µm apart would be desirable, ideally with signal multiplexer integrated in the back end of the probe.

Q: The Buzsaki256 probe is a planar design with 32 sites on each of eight shanks. Do you see advantages to a 3-dimensional probe (with the ability to span both cortical columns and cortical layers) as opposed to the planar approach? Would the 3D approach loosen the density requirements, or would that remain the same?

3-d probes have merits. However, the number one rule in neurophysiology is to record from a healthy brain. Inserting an 8-shank 2-D probe requires quite a bit of surgical experience and despite best intentions, recordings are not always successful. The brain is extremely dense with blood vessels and ‘shooting’ a 3 –D probe into the tissue comes with costs. I have seen histology from many such 3-D arrays from rodents to primates and layer I and often layer 2 are invariably destroyed or compromised. Studying such mutilated cortical tissue is not my intention, although I understand that recording from units even from such compromised tissue can be useful for certain applications.

Q: Your lab has utilized the Buzsaki256 probe design in a published study entitled “Spatially Distributed Local Fields in the Hippocampus Encode Rat Position” that helped illustrate the need for large spatial coverage. Ultimately, what types of questions do high-channel, high-site-density probes allow you to answer that couldn’t otherwise be addressed?

Silicon probes have the great advantage for studying the mesoscopic local field potential signals. The 256-site probe was designed to accomplish this, allowing to record simultaneously the LFP signals from multiple regions, simultaneously with units (even if the unit yield with such probes is less powerful than from higher density version). Imaging methods have only one type of signal. Electrically one can record both neuronal output with spikes and their inputs with high resolution LFP, as well as looking at the relationship between individual components and their aggregate responses.